华北克拉通南缘洛峪群-汝阳群属于中元古界长城系——河南汝州洛峪口组层凝灰岩锆石LA—MC-ICPMSU—Pb年龄的直接约束

运用LA—MC—ICPMS方法,对河南汝州阳坡村附近洛峪口组中部层凝灰岩夹层开展了锆石U-Pb同位素年代学研究,获得了1611±8Ma的高精度年龄。这一年龄第一次精确标定了该地区洛峪口组的形成时限,并显示该组顶界应接近1600Ma。由于洛峪口组位于华北克拉通南缘原划归“新元古界青白口系”洛峪群的最顶部,洛峪群又覆于“中元古界蓟县系”汝阳群之上,因此,这一新的年代学进展实际上同时也将洛峪群和汝阳群都下压到了中元古界长城系,并将洛峪群顶界限定为该地区长城系与蓟县系分界。结合区域资料,特别是熊耳群(下伏于汝阳群)火山岩近年来的年代学标定(多集中于1750~1780Ma),可初步将该地区汝阳群一洛峪群的形成年代限定为1750~1600Ma之间,对应于国际固结纪(Statherian。1800~1600Ma)即中国长城纪中晚期。华北南缘洛峪口组形成年龄的直接约束及相关地层划分的重新厘定,为中元古代华北克拉通南北缘的准确对比及其与哥伦比亚超大陆关系、早期生命演化等重大地学命题提供了新的重要的年代学证据。

Luoyu and Ruyang Group at the South Margin of the North China Craton (NCC) Should Belong in the Mesoproterozoic Changchengian System: Direct Constraints from the LA-MC-ICPMS U-Pb Age of the Tuffite in the Luoyukou Formation, Ruzhou, Henan, China

By means of the LA-MC-ICPMS zircon U-Pb dating, the high-precise dates of the volcanic eruption, 1 611 ±8 Ma, of an interbedded tuffite layer in the middle part of the Luoykou Formation at the Yangpo Section, Ru-zhou City, Henan Province, has been obtained. With this dates, the depositional age of the Luoyukou Formation can be calibrated accurately as the first time, which indicates that the top age of this formation should be most likely about 1 600 Ma. Moreover, being at the uppermost of the Luoyu Group of the conventional Chinese Neo-proterozoic Qingbaikouan System (1 000 - 800 Ma), which overlies on the Ruyang Group of the Mesoproterozo-ic Jixian System (1 600 - 1 000 Ma), at the south margin of the North China Craton (NCC), the new dating achievement of the Luoyukou Formation will result in a major revision in chronostratigraphy that both of the two groups should be lowered down to the older Mesoproterozoic Changchengian System (1 800 - 1 600 Ma). In ad- dition, it also indicates that the top surface of the Luoyukou Formation might be the operable physical boundary between the Changchengian and Jixian System in this area. On the other hand, combined with the regional data, especially the recent chronostratigraphical dating of the volcanic rocks-dominated Xiong' er Group, which shows that they were erupted mainly from 1 750 Ma to 1 780 Ma in age and superimposed by the Ruyang Group, the depositional time span of both the Luoyu Group and the Ruyang Group can be limited roughly into the period from 1 750 Ma to 1600 Ma. It can correlate with the middle-late portion of the Mesoproterozoic Statherian Period (1 800 - 1600 Ma) internationally, and that of the Changchengian Period in China. The direct constraints of the 1611±8 Ma zircon U-Pb age of the tuffite bed in the Luoyukou Formation and the major revision of the chro- nostratigraphical subdivision at the south margin of the NCC provide a up-to-date and critical timescale for some major ongoing research issues of the NCC during the Mesoproterozoic Era, such as the correlation between the northern and southern margin of NCC, the relationship between the development of the NCC and the break-up of the Columbia Supercontinent, the evolution of the early life on the Earth in the deep-time, and so on.